In heavy metal fabrication industries, it is often required to cut large sheets of metal, typically steel, to a given profile according to a particular design. These sheets may be too large and too heavy for convenient manipulation by hand. In the past, complex patterns were scribed, drawn in chalk or painted on the metal sheet to be cut. Personnel would then, typically, use torches to cut the desired profile as laid out. This system had a number of disadvantages, most notably the reliance on human skill both for transferring the design to the metal sheet, and in making the cut.
More recently, metal plates have been mounted on supports and moved beneath a bridge or gantry having a movable cutting head. The combination of being able to move the bridge on rolling supports in one direction, while being able to move the cutting head across the span of the bridge in a perpendicular direction permitted a profile of arbitrary planar shapes to be cut as desired. The addition of programmable control increased both the accuracy and productivity of the cutting process, and the use of a plasma arc cutting head permits the cuts to be made with higher accuracy than with a flame cutting torch.
In particular, plasma arc cutting has been used with the work piece to be cut supported on a submerged bed located in a bath. The bath is thought to absorb or reduce the noxious fumes produced in the cutting process. When the cut is finished, the liquid level in the bath is reduced to expose the parts, the cut parts are removed, and new materials are added.
However, while there may have been a significant investment in the cutting machinery, and in the controls used to operate the cutting machinery, the portion of the duty cycle spent in the cutting operation remained relatively low. For example, in the first step the supporting frame would be loaded with large sheets to be cut. This loading process generally involved using cranes to lift new sheets from a stock of sheets, carrying the sheets overhead, and carefully placing the new sheets to be cut in position.
Once the sheets were in position, the level of liquid in the bath was raised again to submerge the material. The cutting apparatus could then cut the sheet into the desired pieces. When finished, the cutting head would be moved to one side to give access to the various pieces. The shop personnel would then transport the cut pieces to the next production stage (or inventory, as might be), and remove the scrap. Removal of cut pieces did not tend to occur while other cutting was underway, since it might not have been prudent for personnel accidentally to provide an unintentional path to ground for the plasma arc, loading generally requires the lowering of the liquid level in the bath, and a mistake in moving material could result in shifting the remaining pieces to be cut, thus possibly yielding an incorrect profile. For these reasons it was generally only practical to remove pieces after cutting was complete.
Unloading commenced with the lowering of the level of the bath. Once all of the useful pieces and scrap had been moved away, the new stock could be brought in, and the table re-loaded. Typically, the removal of cut pieces and scrap impeded the placement of new sheets on the cutting frame such that removal had to be complete before the laying of new sheets for cutting could begin. Once loaded with new material, the level of the bath would be raised again to immerse the new sheets. All the while, during loading of new stock and unloading of cut pieces and scrap, the cutting machinery tended to be idle. By one estimate, the length of time spent loading and unloading significantly exceeded the actual time spent cutting. Although it was possible to improve production by operating two baths end-to-end, sharing a cutting head, the overall level of productivity was not necessarily entirely satisfactory. It would be advantageous to cut material more nearly continuously, and to permit at least some of the unloading of cut parts to occur in a different location, from the loading of new sheets of metal.
The baths had another disadvantage related to operation in cooler climates. Although sheltered from wind and snow, the shop was typically not heated. If the bath were left inoperative for a significant length of time, such as during Christmas shut-down, either a heating element was required to keep the bath above freezing, or anti-freeze was required, or both. Antifreeze would have to be drained, and the bath flushed before recommencing operation. If the bath were allowed to freeze, the time and effort required to put the system back in operation was significant.
In still more recent times, plasma arc systems have been operated without using the submerged bath apparatus. Rather than using a liquid medium to absorb undesirable gases, a vacuum system is used to draw off the gases. Not using a bath system provides the opportunity to unload and reload the work material not piece-by-piece, while the cutting heads sit idle, but rather by changing out the entire bed. That is, by having several beds, and moving them relatively quickly, the entire cutting facility can be re-loaded in a few minutes, and then loading and unloading can take place elsewhere while the material in the next bed is cut. When a non-liquid bath system is used, it is possible to move an entire bed more easily, without concerns about managing the liquid in the bath. The beds can be moved either by lifting, as with overhead gantry cranes, or by movement along a track work, such as might be laid on the floor of a bay.
In the former process, parts were removed piece-by-piece, using electromagnetic clamps to lift the cut pieces of steel. (in the case of non-magnetic materials magnetic lifting is not appropriate, in which case other lifting techniques, such as suction systems can be used in some instances). It would be advantageous to lift all the pieces off the bed at once, or in a relatively small number (two or three) of lifts, then to carry them away with an overhead crane, or to lay then on pallets where other equipment, such as forklifts and tractors, can sort and transport the cut parts as need be. It is similarly advantageous to be able to re-load the beds in a position away from the cutting head, or heads. By doing this, a vacant bed can more quickly be made ready for another cutting operation.
Use of a bath to submerge the material to be cut tended also to impose practical limitations on the size of pieces that could be cut. Part size was limited by the bath size. When a particularly large panel was desired a number of individual plates would have to be cut, aligned, and butt-welded together. For example, a number of different types of rail road cars employ side walls that are of the order of 60 ft in length, and up to about 11 feet in width from side sill to top chord. An example of such a car is a grain car or a plastic pellet car. These sidewalls have generally been fabricated by joining a plurality of plates together. It would be advantageous to be able to accept sheet from the rolling mill in a sufficient length and width to be able to cut these side walls from a single sheet, thereby eliminating the fabrication involved in butt-welding a number of plates together. Such a method of fabrication would tend to reduce defects in the resultant car structure, would tend to reduce tolerance build-up in the overall assembly, and generally facilitate assembly of the cars.
At present, long steel sheet, such as would be required to make a single piece side sheet of 60 ft length is available in coils from the rolling mill. To process the sheet directly, the coils require uncoiling. Consequently, it would be advantageous to provide a de-coiling facility adjacent to the cutting facility. It would also be advantageous to be able to load the uncoiled sheet directly into a cutting bed, and then move the bed into position for cutting.